^{The tiny ranching town of Parkfield is California's seismic playground for scientists. This instrument package sits in Turkey Flat. Photos by Andrew Alden.}

Besides earthquakes, there is a whole spectrum of energetic activity in the solid Earth. Thankfully, most of it doesn't disturb anyone's sleep. After a century of focus on jolts and jiggles, earthquake scientists have begun turning their attention to these more subtle signals. And California is one of the prime laboratories for this research.

Most big earthquakes happen in the mid-crust in a zone between 10 and 20 kilometers deep, where rocks are strongest. Above this zone, rocks are cold and brittle and tend to crack; below, they are hot and ductile and tend to stretch. Menlo Park seismologists looking at the deeper, ductile crust have put a new piece of the great puzzle into place this week.

Earthquakes of the classic type—cracks in the ground, alarums and mayhem in the human sphere—are only the best known type of seismic activity. They center around the brittle–ductile transition zone, but can be found from very near the surface down to almost 100 kilometers deep, if tectonic forces have put cold, brittle rock down there. (Deep earthquakes, which occur down to almost 700 kilometers, are a separate species.)

The new types of seismic events are small and slow murmurings within the deep crust. They involve motions that are gentler and at much lower frequencies than typical earthquakes. Think of them as less like the crisp cracking of a baguette crust and more like the quiet ripping of the bread inside.

Different varieties of these slow events have been discovered by different people in the last two decades. Their names include tremor, slow earthquakes, episodic slip events, transient creep and so on. They are hard to detect and difficult for seismologists to model. But that's what they used to say about ordinary earthquakes, and I'm sure that we will tame these creatures too. I suspect that they will eventually align in a kind of spectrum with names akin to the colors we designate in the blurry bands of the rainbow.

This week's news centers on the lively research topic of triggering: Do distant earthquakes set off local events as their seismic waves roll through? It seems obvious that they would, but science looks for proof before accepting even what seems obvious. Triggering was first accepted when the 1992 Landers earthquake in Southern California was shown to set off small quakes all the way out to Yellowstone, in Wyoming. The effects are subtle and of scientific rather than engineering interest.

The San Andreas fault is a laboratory for slow-event research; the area around Parkfield, east of Paso Robles, has been intensively instrumented since the 1970s. Recently, persistent clusters of tremor have been mapped there at depths below the earthquake zone. A paper by U.S. Geological Survey seismologist David Shelly and others published this week in Nature Geoscience notes that these ticklish spots of "ambient tremor" are sensitive to large, distant earthquakes in the right circumstances.

_{Frame from Supplementary Movie 3, Shelly et al., Nature Geoscience doi:10.1038/ngeo1141. North-south profile of San Andreas fault; shaded zone and star represent slip in and hypocenter of the 28 Sept 2008 Parkfield earthquake; blue dots are earthquake events, crosses are ambient tremor locations; depth grid is 10-km intervals.}

Here's an example grabbed from a Quicktime movie file showing these ambient tremor spots "lighting up" after passage of the surface waves from the February 27 2010 Chile earthquake (magnitude 8.8). Surface waves are the strongest, most damaging and slowest-moving vibrations of an earthquake; they are what makes the entire planet reverberate as long as weeks afterward. What interested the researchers about this triggered tremor is that it lasted long after the triggering waves had passed. Something was moving very slowly after the trigger left, something that continued to set off tremor until the stresses finally dissipated.

What moves slowest of all the new seismic slow events? It is creep, which I described in last week's post. Creep is known to be variable—some parts of a fault have it while others don't; its speed also varies in different locations. It's known to change speed, too. In fact, you might think of it as just an extremely slow earthquake with a frequency of years instead of seconds.

Shelly and his coauthors therefore suggest a new kind of triggered event to go along with triggered earthquakes and triggered tremor: triggered creep.

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^{Suburbia is warped in downtown Hayward, where fault creep is steadily distorting curbs, walls and pavement. All photos by Andrew Alden except where noted.}

In my first post on KQED Quest Science, I invited you to make friends with your local earthquake faults. The best place to do so, in my opinion, is in Hayward, where the Hayward fault runs right through downtown. You can start at the Hayward BART station and enjoy a good meal while you're in town.

What makes Hayward special is that the fault moves steadily there, even without earthquakes, in the process that geologists call aseismic slip, or fault creep. Very few faults are known to creep, but in the Bay Area we have three: the Hayward fault, the Calaveras fault in Hollister, and the San Andreas fault south of Gilroy.

I love my copy of U.S. Geological Survey Map MF-2196, "Recently Active Traces of the Hayward Fault," published in 1992 on a topographic map base. But the Survey's online database, updated twice since then, costs nothing and is projected on aerial photos. Here's how downtown Hayward looks in it; click the image for a larger version. North is to the left.

The BART station is at the bottom between A and D Streets. If you can, walk through the new City Hall on your way to Mission Boulevard; it's highly engineered for earthquake resistance, and the apron of stone around it hides a base-isolation system that lets the building shimmy like a surfer to ease the stress on it from a magnitude-7 event.

On Mission it's simply a matter of walking in either direction and going up each side street. The annotations have a simple code: C1 and C2 denote excellent and good creep evidence, respectively. Here are examples of the codes that follow them. First is "ec," for echelon cracks, like this set from Oakland's Lake Temescal where the fault has its own exhibit.

Before we go any farther I should point out that Hayward is not proud of its fault. Many homes and properties were built upon the fault trace before creep was first recognized in 1956. It's best not to stand and point. Geology teachers tell their classes the same thing.

The code "rc" stands for right-offset curb. These are easy to spot. I photographed this example a few years ago; since then the construction site has become a structure, but the curb still takes its rightward jog. The Hayward fault is classified as right-lateral, meaning that when you look across it, the other side moves to the right.

The curb on Sunset Street, near Prospect Street, is a clearer example.

The codes "rb" and "rw" mean offset buildings and walls, respectively. You'll see a lot of damaged brick buildings, or recent repairs, in line with the bent curbs. The code "jo" means open joints or cracks in concrete. This example shows a steadily opening gap that has been patched.

Localities marked "so" have had their offsets surveyed. The most commonly visited site is the corner of Rose and Prospect Streets, the yellow circle on the map. The corner curb has had a constant stream of visitors since it was built in 1971. This is how it looked in 2001.

_{Photo courtesy misspudding of Flickr under Creative Common license}

You can see from my photo taken in 2007 that it has moved further since then. The street here has a good set of echelon cracks too.

Prospect Street got its name because it runs along a low ridge with a nice view west. The ridge is a pressure ridge, thrown up during thousands of years of creep and earthquakes. That's one natural sign of fault activity. Another is offset streams, which are just like offset curbs only much larger. You'll see one on the map where a stream flows down from the top (east), turns sharply to its right, then jogs leftward on the other side of the fault. It's marked "G1, rs" meaning excellent geomorphic evidence consisting of a right-offset stream.

Another natural fault sign appears about 2 miles south on Mission at Holy Sepulchre Cemetery, where you can look up on the hillside just to the south and see another natural sign of the fault: a line of springs. It really stands out in the dry season.

This is just a beginning of what geologists learn to see, but you can follow in their footsteps as far as you care by visiting the USGS's Northern California earthquake home.

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New instruments hook to the underwater lab.
Credit: David Fierstein © 2005 MBARI

We heard about the Monterey Bay Aquarium Research Institute's new underwater laboratory in a radio story last fall. When that story aired, the lab (known as the Monterey Accelerated Research System, or MARS) was just getting going, with lots of neat experiments planned. Now, few of those have become a reality.

In case you missed the first story, the MARS is essentially an underwater data hub, perched on the ocean floor almost 3,000 feet below the surface of Monterey Bay. A 32-mile cable connects the system to land, acting as a power cord and data link. Several "underwater extension cords" allow a variety of instruments to plug into the hub, getting power from land and sending back data via the cable. That constant connection is a big step forward in undersea science; without it, researchers have had to use boats to stay physically close to their instruments (something hard to do for very long), or have sent the instruments off on their own, relying on batteries to keep them running and collecting data.

Until late February, earthquake scientists at the UC Berkeley Seismological Laboratory had been using that second method with their seafloor seismic station, the Monterey Ocean Bottom Broadband (MOBB). "We had to wait three months to even know if the instruments were alive," said Barbara Romanowicz, the lab's director. But the MOBB is now plugged in to the MARS system, and is transmitting its information about earthquakes in real-time.

That new stream of information could be especially valuable in California, because the MOBB provides a unique view of the main fault system, the San Andreas, which runs along the Northern California coast. Most seismometers are land-based, and therefore positioned on the east side of the fault. The MOBB is on the west side of the fault, offering a helpful perspective on the fault's shifts and shakes.

The researchers hope that the MOBB's new stream of real-time data will improve their earthquake models, and perhaps eventually help provide early warnings about impending quakes (for more on that topic, see the TV story, Earthquakes: Breaking New Ground).

The MOBB is just one instrument using the MARS hub. A tool that uses sound waves to track fish is currently attached, and within the next six months you can expect to see a robotic DNA lab and a robot that crawls along the seafloor, collecting data on animals that live in the mud.

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Updated: On March 11, 2011, a massive 8.8 magnitude earthquake hit the Pacific Ocean nearby Northeastern Japan. The quake, and a tsunami that followed, caused massive damage and loss of life. The news put quake prone California on alert. While many of us would rather not think about the possibility of another major quake, we are surrounded by active faults. One East Bay fault has scientists especially concerned.

It's been called the most dangerous fault in the U.S. The Hayward Fault runs 40 miles, from San Pablo Bay to Fremont, through some of the most densely populated areas in the country. Every 140 years for the past two thousand the Hayward Fault has jolted the East Bay. Geologists have figured out the regular history of these quakes by carbon dating trenches along the fault. A lesser known cousin of the San Andreas the Hayward fault is a creeper. Basically, it moves, slowly, along the surface but deep inside… it's locked until tension builds up and and it slips. It appears that it is time for the fault to slip again. The last major earthquake on the Hayward fault was 1868. Scientists believe that the temblor registered 7.0 in magnitude. Hayward and San Leandro were devastated. But if the quake were to happen today, it would be a much different story.

I met Mary Lou Zoback out at the Fremont Bart station, which sits right on top of the Hayward Fault. She pointed out cracks in the parking lot from the creeping fault. Zoback is a geophysicist who worked 28 years at the U.S. Geological Survey and who has done catastrophe modeling of risky residential buildings. Her company estimates that a 6.8 quake, or bigger, on the Hayward Fault could cause a disaster on par with Hurricane Katrina, causing 168 billion dollars in damage and leaving at least 200,000 homeless.

A number of public buildings in the east bay are undergoing retrofitting to make them more structurally sound. Area hospitals have until 2013 to meet seismic safety standards. There is a state inventory of public schools prone to collapse in a major quake, but no such list exists for private schools. And retrofitting standards for risky residences are confusing. I talked with Jim Cook, of Bay Area Retrofit. He says existing codes are unclear and there really is no specific licensing for seismic home retrofitters. Cook has been fighting local governments for years to improve seismic safety standards.

Homeowners can have their home evaluated but what if you are a renter? Many apartments and condos can collapse in earthquakes because they have parking or open commercial space on the first floor making this story weak or "soft." According to the Association of Bay Area Governments Earthquake and Hazards Program, soft-story apartment buildings were responsible for about two-thirds of the 46,000 uninhabitable housing units in the 1991 Northridge earthquake. In the Bay Area, unreinforced masonry (older buildings constructed of brick, stone or cement blocks) continues to be a threat.

The thought of a big earthquake is scary enough, never mind the chaos that can happen in the aftermath. But the damage from a large earthquake has repercussions that can last for a very long time. We can still see the scars from the 1989 Loma Prieta earthquake. Downtown Santa Cruz is not yet fully rebuilt and retrofitting continues on the Bay Bridge. We can prevent a lot of damage up front by shoring up our buildings and creating a family disaster plan and an earthquake kit. The Hayward Earthquake Alliance has put together some really helpful information on how to prepare for a major quake.

Listen to the Hayward Fault Radio Report and view the recent QUEST TV segment on the fault online.

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Can elephants feel seismic waves?

Scientists have known for years that elephants can communicate. By using low frequency vocals, called rumbles, elephants can 'talk' with eachother, sometimes communicating from very long distances.

But the new question being asked by some scientists is: can elephants feel those rumbles in the earth?

Biologist Dr. Caitlin O'Connell-Rodwell from the Oakland Zoo wants to find out. After studying elephant activity in Africa, she noticed that elephants would raise and lower their feet when interacting with one another. She realized that these elephants were using seismic waves felt through their feet to send messages.

O'Connell-Rodwell and her team have been creating mini-earthquakes for an elephant (named Donna) at the Oakland Zoo to monitor her responses to different seismic activities.

Check out this National Geographic video about the study on YouTube:

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